1. Field of the Invention
The present invention relates to a wireless communications system. More particularly, the present invention relates to synchronizing hyper-frame numbers during a SRNS relocation procedure in a 3GPP wireless system.
2. Description of the Prior Art
As explained in the 3rd Generation Partnership Project (3GPP) specifications 3GPP TS 25.322 V3.10.0 “RLC Protocol Specification”, and 3GPP TS 25.331 V3.10.0 “Radio Resource Control (RRC) Specification”, which are included herein by reference, serving radio network subsystem (SRNS) relocation is performed to transfer user equipment (UE) from one radio network controller (RNC) to another RNC. Various reconfiguration procedures may involve SRNS relocation, which is indicated by certain information elements (IEs) included in the reconfiguration message.
When receiving a reconfiguration message indicating that SRNS relocation is to occur, the UE re-establishes the radio link control (RLC) entity of radio bearer RB2, and re-initializes the uplink and downlink hyper frame number (HFN) components of the COUNT-Cs for RB2 with MAX(uplink HFN of RB2, downlink HFN of RB2)+1, where MAX(a, b) gives the larger of a and b. The UE also calculates the START value for each core network (CN) domain according to subclause 8.5.9 of the RRC specification TS 25.331 V3.10.0, and includes these START values in the IE “START list” in the response message.
The UE then transmits the response message to the UTRAN on RB2, thereby providing the UTRAN with most recently transmitted START values. After the RLC has confirmed the successful transmission of the response message, the UE re-establishes the RLC entities of other radio bearers and re-initializes the HFN of the COUNT-C values for each radio bearer with the START value included in the response message for the corresponding CN domain.
As described in the prior art, in the event that SRNS relocation is involved in a reconfiguration procedure, the UE shall re-establish the RLC entities of RB2 and re-initialize the HFN of the COUNT-C for the radio bearer before transmitting the response message.
For the Target RNC to be able to decipher the response message sent by the UE on RB2, the UL COUNT-C of the UE and the UTRAN have to be initialised with the same value, i.e. MAX (uplink HFN component of COUNT-C of RB2, downlink HFN component of COUNT-C of RB2)+1. The UL HFN and DL HFN of RB2 are provided to the Target RNC by the Source RNC in an SRNS Relocation Info container. The data in the SRNS Relocation Info container is forwarded via the CN. There is a possibility that between the time at which the Source RNC evaluates the HFN and the instant when the UE does so, some RRC messages may be sent on the UL of RB2 (for example, Measurement Report messages). If the UL HFN of RB2 is greater than the DL HFN of RB2, and if the RLC sequence numbers on the UL of RB2 are close to roll over, such messages may trigger an HFN increment in the UE but not in the UTRAN, as those messages might not have been considered when the UL HFN is sent from the Source RNC to the Target RNC. Under such circumstances, the COUNT-C of RB2 will not be initialised with the same value in the two entities (i.e., the UE RLC entity and the Target RNC RLC entity). If this happens, the Target RNC will be unable to correctly decipher the response message, and the SRNS relocation procedure will consequently fail.
The SRNS relocation may also be initiated by a cell update procedure, and in which case the above-noted problem continues to exist.